What are the proofs that the Noble Qur'an has not been altered, and what is the response to the doubt that the letters and vowel marks came after Islam?

It is not permissible to access websites of enemies of Islam if you do not possess knowledge to repel misconceptions, for Allah the Almighty says: "And do not throw [yourselves] with your [own] hands into destruction." The Sunnah (definition) has indicated the importance of staying away from doubtful matters. The Qur'an is preserved from addition and reduction, for Allah the Almighty says: "Indeed, it is We who sent down the message [i.e., the Qur’ān], and indeed, We will be its guardian," and His saying: "And indeed, it is an honorable Book. Falsehood cannot approach it from before it or from behind it." The Qur'an has been transmitted through tawatur (multiple, successive narrations), so its transmission relied on memorization in chests, not on copies of the masahef. Arabic letters are as old as the language itself, and there were Companions who wrote down the Qur'an. As for diacritics (vowel marks and dots), they were written after the era of the Prophet, peace be upon him, due to the Arabs' lack of need for them at that time, and this does not affect the authenticity of the Mus'haf.